Abstract
Asynchrony among synaptic inputs may prevent a neuron from responding to behaviorally relevant sensory stimuli. For example, “octopus cells” are monaural neurons in the auditory brainstem of mammals that receive input from auditory nerve fibers (ANFs) representing a broad band of sound frequencies. Octopus cells are known to respond with finely timed action potentials at the onset of sounds despite the fact that due to the traveling wave delay in the cochlea, synaptic input from the auditory nerve is temporally diffuse. This paper provides a proof of principle that the octopus cells' dendritic delay may provide compensation for this input asynchrony, and that synaptic weights may be adjusted by a spike-timing dependent plasticity (STDP) learning rule. This paper used a leaky integrate and fire model of an octopus cell modified to include a “rate threshold,” a property that is known to create the appropriate onset response in octopus cells. Repeated audio click stimuli were passed to a realistic auditory nerve model which provided the synaptic input to the octopus cell model. A genetic algorithm was used to find the parameters of the STDP learning rule that reproduced the microscopically observed synaptic connectivity. With these selected parameter values it was shown that the STDP learning rule was capable of adjusting the values of a large number of input synaptic weights, creating a configuration that compensated the traveling wave delay of the cochlea.
Highlights
A singular sensory event such as a flash of light, the sound of a clap, or the onset of a glottal pulse in speech may create a neural response that is temporally diffuse
The “genetic” search algorithm was used to find the parameters of the homeostatic rule and the spike-timing dependent plasticity (STDP) rule that best achieved the observed configuration of synapses for the octopus cell
It was found that the algorithm could select synaptic weights such that the dendritic delay compensates for the traveling wave delay (Figure 4A)
Summary
A singular sensory event such as a flash of light, the sound of a clap, or the onset of a glottal pulse in speech may create a neural response that is temporally diffuse. The combination of input asynchrony combined with their intrinsic capacity for coincidence detection appears to be paradoxical This incongruity is resolved if octopus cells utilize their dendritic delay to compensate for the artificial asynchrony across ANFs (McGinley et al, 2012; Spencer et al, 2012). This hypothesized configuration would require very accurately located synapses so that the post-synaptic potential’s (PSP) propagation delay along the dendritic tree matches the differential delay between ANFs of differing CF. It has been noted (Golding and Oertel, 2012) that the mechanism of development and regulation of such a finely balanced synaptic configuration is yet to be understood
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